Kunststoffe 71 (1981), No. 1, pp. 18-26 discloses degassing devices in which an insert member is incorporated in the interior space of the extruder housing for degassing the plastic material in multi-screw extruders, the opening in the insert member providing communication between the interior of the housing and a degassing chamber for removing separated gas. The degassing chamber is arranged on the upper side of the housing and requires a special configuration, based on the rheological behavior of the plastic melt and/or the order of magnitude of the process parameters in the screw extruder. This arrangement has the undesirable result that separated melt as well as condensation products can reach the primary flow of melt being processed, which then becomes contaminated.
A degassing device for a single screw extruder is disclosed in Chemie. Ing. Technik 1962, No. 11, p. 751 in which the degassing device is arranged laterally downwards. This device is suitable only for masses of a certain low viscosity.
In another extruder disclosed in DE 37 03 758A1, the degassing opening is connected to a vacuum chamber in the region of the side wall of a cylindrical screw housing at an angle of inclination relative to a line normal to the axis of the housing, so that plastic melt discharged from the cylindrical screw housing is guided back into the plastic material being processed in the housing.
The known degassing devices have the disadvantage that they tend to introduce separated quantities of plastic or condensation products back into the main stream of the plastic melt, especially for high flow in the degassing channel. Particularly serious in the case when product residues entrained in the gas flow are baked in the degassing channel causing contamination of the melt product being processed due to decomposition of the residues because of the longterm temperature effect.
Where this is not the case, such as, for example, in screw housings having degassing openings directed laterally downwards, only plastics within narrow viscosity limits can be processed, so that extensive retooling of the screw extruder is necessary for the degassing of different polymer solutions or melts.